I did a Coolermaster HAF 932 case mod last year. I painted the interior flat black. Matte black would be good too. I saw a pc case at a Fry's Electronic's store with an interior that was painted gloss black. It didn't look that good.
I cleaned the interior first and sprayed it with metal primer. Then I sprayed it twice with flat black paint. I used Rustoleum primer and paint available at any hardware store.
The exterior paint job was different. It was a two-tone paint job - black and copper. For the copper I used white primer (brings out the metalic look better), metallic copper paint, and a clearcoat over the copper. The combination matched real copper panels that I used fairly well. I bought the primer and paints at a hobby shop because I was also painting the front bezel which is plastic. The hobby shops have primer and paints especially designed for plastic. Do not use regular spray paint. It will eat the plastic.